The instant invention relates to metal powders in general and, more particularly, to a continuous method for producing submicron sized metal particles. These particles are especially useful for inclusion in slurries and pastes destined for electrochemical cells such as batteries and for capacitors.
As discovered by Mond and Langer in 1889, nickel tetracarbonyl-Ni(CO)4xe2x80x94readily decomposes into essentially pure metallic nickel and carbon monoxide within the temperature range of about 150-315xc2x0 C. A major drawback of the exquisite Mond process is that nickel tetracarbonyl is a highly dangerous toxin. Similarly, carbon monoxide must be treated with extreme care. Accordingly, few organizations throughout the world employ the method.
The multi-layer capacitor paste market requires submicron (less than one micron) nickel powders with no agglomerated particles larger than about one micron in size.
Most commercial fine nickel powders are made by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d), chloride reduction or aqueous precipitation. These current technologies are very expensive to scale up.
The resultant nickel powders used by multi-layer capacitor manufacturers are initially sold as dry powders. The fine powders are subsequently dispersed into a liquid to form a slurry which is part of the paste making process. Manufacture of the paste adds a significant cost to the final product.
For decades, assignee has decomposed nickel (tetra) carbonyl vapors in the gas phase to manufacture a variety of fine pure nickel powders. Extra-fine powders, with a primary particle size of less than about 0.5 microns, may be made by gas phase decomposition of the nickel carbonyl at temperatures above about 400xc2x0 C. Unfortunately, under these conditions, particle collisions create significant opportunities for sintering resulting in powders containing some undesirable particles in excess of about one micron.
All current techniques for producing dispersions containing submicron particles require expensive multi-step batch operations to manufacture the desired slurries and pastes.
During the early development of nickel carbonyl technology around the turn of the last century, it was recognized that the passage of nickel carbonyl with hydrogen through a fluid resulted in the catalyzation and formation of organic compounds. See German patent 241,823 (dated 1911) to Shukoff.
Similarly, U.S. Pat. No. 1,138,201 to Ellis teaches the hydrogenation of heated oils. The nickel carbonyl is utilized as a source of fine catalytic nickel within the oil.
In both instances, the nickel particles are separated from the liquid leaving behind the hydrogenated compounds. However, apparently there was no recognition that a subsequently treated liquid dispersion, such as a paste or slurry, with entrained nickel particles therein had any utility.
There is a need for a continuous cost-effective process that produces submicron metal powders in liquid dispersions, thereby short-circuiting a number of intermediate processing steps.
There is provided a continuous low cost process for making a liquid dispersion of submicron sized particles of pure metal.
Metal carbonyl vapor bubbles are introduced in a heated liquid with an inert carrier gas. As the bubbles rise, the metal carbonyl decomposes into submicron metal particles without agglomerating while simultaneously dispersing throughout the liquid. The resultant nickel primary particles have an average diameter of about 0.1 microns, an order of magnitude smaller than most current commercial nickel particles.